Effect of 6- Versus 12-Hour Interval Between 2 Prostaglandin F2α Injections Administered with 5-Day CO-Synch + Controlled Internal Drug-Release Protocol on Pregnancy Rate in Beef Cows

The objectives of this study were to compare 2 prostaglandin (PG) F2α injections administered at 6- or 12-h intervals and 5- versus 7-d controlled internal drug-release device (CIDR) insertion on timed AI (TAI) pregnancy rate in beef cows synchronized with CO-Synch protocols. The study was conducted at 2 locations using Angus-based multiparous cows (n = 257 at ranch 1, and n = 385 at ranch 2). Cows were randomly assigned to 1 of 3 treatments: 1) 7CO-Synch (control) cows received gonadotropin-releasing hormone (GnRH) and CIDR on d − 7, PGF2α concomitant with removal of the CIDR on d 0, and GnRH with TAI at 60 h; 2) 5CO-6H cows received GnRH and CIDR on d − 5, PGF2α at 0 and 6 h after removal of the CIDR on d 0, and GnRH with TAI at 72 h; and 3) 5CO-12H cows received GnRH and CIDR on d − 5, PGF2α at 0 and 12 h after removal of the CIDR on d 0, and GnRH with TAI at 72 h. Body condition score and postpartum interval were greater (P < 0.01) for cows on ranch 1 than ranch 2 (4.8 vs. 4.7 BCS and 79.5 vs. 68.1 d). Body condition score, postpartum interval, TAI pregnancy rate, and 70-d pregnancy rate did not differ (P > 0.25) between synchronization protocols, averaging 4.8, 73.8 d, 49.0%, and 58.1%, respectively. The extra time and costs associated with implementing the 5-d protocols with 2 PGF2α injections compared with the 7-d protocol with 1 PGF2α injection were not offset by higher TAI pregnancy rates.